The invention relates to a process for the fractionation and extraction of hydrocarbons and in particular a process for the production of petrol with an improved octane number and kerosene with an improved smoke point and optionally a process for the production of diesel or gas oil with an improved cetane number by the selective extraction of appropriate hydrocarbon fractions.
It is known to produce petrol through atmospheric distillation of a hydrocarbon charge. The liquid effluents distilling between approximately 50.degree. and 130.degree. C. constitute a petrol fraction or cut with a relatively low octane number due to the preponderance of saturated hydrocarbons and to said fraction can be added at least part of a heavy petrol fraction at 130.degree. to 220.degree. C., which is richer in aromatic hydrocarbons and of interest due to its high octane number. This solution is totally adopted in summer, part of the heavy petrol being used for aviation fuel. However, in winter, most of the heavy petrol is mixed with the diesel oil fraction to meet heating requirements.
It is also known to improve the octane number of the petrol fraction by reforming, cf. U.S. Pat. No. 3,044,950, but the nitrogen content of the charge must not exceed 50 ppm, which makes it impossible to directly treat the heavy petrol fractions of 130.degree. to 220.degree. C., which are richer in nitrogen and in particular heavy catalytic cracking petrols.
In order to obviate this disadvantage, it is necessary to pretreat using a severe hydrotreatment said heavy petrol fraction, said pretreatment being difficult and expensive.
Moreover, French Patent No. 1,421,273 discloses a process for the extraction of hydrocarbons (e.g. benzene, toluene and xylene and/or polycyclic aromatic hydrocarbons) using an extraction solvent, such as dimethylsulfoxide, in a first extraction zone. However, its use has come up against numerous difficulties due to its mediocre stability with respect to heat and distillation treatments. In order to avoid distillation during the separation of the mixture incorporating the extract enriched with aromatic hydrocarbons and dimethylsulfoxide, it is advantageous to use an auxiliary solvent in a second extraction zone, which is able to dissolve the hydrocarbons of the charge, but unable to dissolve a substantial quantity (more than 5% of its weight) of dimethylsulfoxide. The latter can consequently be separated and then recycled into the first extraction zone.
In order to recover this extracted and substantially purified aromatic fraction, it is necessary to carry out a number of operations such as washing with water in order to remove the dimethylsulphoxide traces, followed by a distillation of the remaining mixture, i.e. the aromatic hydrocarbons and auxiliary solvent, which is thus separated and then recycled into the second extraction zone.
Moreover, French Patent No. 1,424,225 teaches the extraction of aromatic and non-aromatic constituents of light cycle oil (L.C.O.) constituted by a distillation fraction of approximately 204.degree. to 316.degree. C. produced by catalytic cracking of the petroleum in two successive zones. In the first, use is made of a mixture of dimethylformamide and approximately e.g. 10% water as the solvent for the aromatics, while in the second, use is made of a mixture of saturated hydrocarbon-rich naphtha and xylene in order to extract a mixture containing the aromatic hydrocarbons and recycle the solvent to the first extraction zone.
The thus extracted mixture is then passed into a distillation column. At the bottom or tail an aromatic hydrocarbon concentrate is collected and at the head at least part of the naphtha is collected which is then recycled into the second extraction zone. This operation is expensive from the energy standpoint.
The prior art is finally illustrated by U.S. Pat. No. 3,044,950, which describes a double treatment of a 25.degree.-220.degree. C. charge combining a hydrotreatment, followed by an extraction by solvent. This hydrotreatment is performed on the entire charge, or on the heaviest fraction of the charge obtained by distillation.
The hydrogenation of the lightest fraction of the charge leads to an octane number reduction and the hydrogenation of the heaviest fraction can only be carried out under very severe pressure conditions in view of the large nitrogen quantity in said fraction. Moreover, the hydrogenation of the heaviest fraction and consequently the aromatic hydrocarbons contained therein leads to a reduction in its octane number.